Flow Battery Electroanalysis: Hydrodynamic Voltammetry of Aqueous Fe(III/II) Redox Couples at Polycrystalline Pt and Au

The redox flow battery (RFB) is a promising technology for large-scale electrochemical energy storage, but research progress has been hampered by conflicting reports of electron-transfer rates even for well-established battery chemistries. To address this challenge, we are working to deploy established electroanalytical techniques for precise characterization of RFB reaction kinetics. We studied Fe^3+/2+ redox chemistry using rotating-disk electrode voltammetry with polycrystalline Pt and Au working electrodes as a model of an Fe/Cr RFB positive electrolyte. Our measurements yielded exchange current densities of 3.7 ± 0.5 and 1.3 ± 0.2 mA cm^–2 for Pt and Au, respectively, in electrolytes containing 5 mM each of Fe³⁺ and Fe²⁺. Both the variability and relative sluggishness of these rates are clear evidence that inner-sphere (catalytic) processes are important even in the 1-electron redox chemistry of Fe aquo complexes. Increasing the Fe concentration by 100-fold gave exchange current densities at Pt that were only ∼15-fold higher, suggesting that the reaction is not first-order in Fe or that the predominant mechanism changes as electrolyte concentration is increased. These results motivate further studies of RFB electrocatalysis under conditions that prioritize both analytical precision and device-level applicability.